Lithium ion secondary batteries are provided with positive and negative electrodes which absorb and release lithium ions, a separator interposed therebetween, and a non-aqueous electrolyte. The negative electrode usually includes a negative electrode core material sheet such as copper foil, and a negative electrode material mixture layer supported on a surface thereof. Such a negative electrode is formed by, for example, applying a negative electrode material mixture slurry including a negative electrode active material and a binder onto a surface of a negative electrode core material sheet, followed by drying, and then pressing between rolls. The pressing increases the density of the negative electrode material mixture layer. The negative electrode material mixture slurry is prepared by, for example, dispersing negative electrode active material particles and a binder in a dispersion medium such as water.
In lithium ion secondary batteries, grooves or cup-shaped recesses have been formed on a surface of the active material layer in order to solve various problems.
For example, Patent Literature 1 discloses forming grooves or cup-shaped recesses by pressing a die to a surface of the active material layer. When a lithium ion secondary battery is used continuously at high temperatures, the separator is softened, and the protrusions and recesses on its surface are deformed, resulting in reduced space for retaining non-aqueous electrolyte, and thus in deteriorated high-output characteristics. Patent Literature 1 teaches that this problem can be solved by the above method.
Patent Literature 2 discloses forming grooves which extend to the edges of the surface, on a surface of the active material layer, so that penetration of non-aqueous electrolyte into the electrode group during production can be improved. Such grooves for improving penetration of non-aqueous electrolyte into the electrode group, however, reduce the strength of the electrode. Patent Literature 3 discloses forming lattice-like grooves on a surface of the active material layer, for the purpose of solving this problem.